As a result, “many people don’t know what syphilis is or how it’s transmitted,” said Darryl Richards, the STD coordinator in Decatur, Georgia, which has one of the highest syphilis rates in the country. “Most people associate sexually transmitted infections with a discharge or a burning sensation. However, syphilis does not cause these symptoms.”

Among gay men, the syphilis infection rate has increased to levels not seen since the start of the AIDS epidemic of the 1980s. Some public-health experts we interviewed think the syphilis resurgence might be driven, in part, by the rise of hook-up apps such as Grindr. And with the AIDS crisis now a distant memory, gay men might not be as careful with condoms as they once were. Indeed, those factors help explain why syphilis rates have also spiked in parts of Canada and Europe.

But others say a big reason is federal and state governments’ failure to adequately fund local public-health budgets.

“This is a sentinel event—it is a failure of the healthcare system,” Gail Bolan, the CDC’s director of STD prevention, said at a recent Congressional briefing.